Hello, I'm Dr. Deb.
I recently came online and see that your question about Cowie hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
I share your concern about Cowie given her behavior. Not all kittens will gain about 1 pound a month although most of them do (as you've seen with Cowie's brother).
So, her weight is worrisome.
Some kittens are pretty laid back and don't play a lot but they don't tend to sleep as much as she is at this age.
Her appetite should also be better than it is.
Unfortunately, I'd be worried that she may have an underlying birth defect or possibly a viral infection such as Feline Leukemia or Feline Infectious Peritonitis (FIP) although it gives me no pleasure to say so:(
Leukemia can often be diagnosed with a blood test which is run in a vet's office. FIP is sometimes a tad more difficult to diagnose but if she has abdominal fluid (which is why she has a belly but is skinny elsewhere), it can be evaluated. These kittens are also usually running a fever so you can try and take her temperature at home; normal is between 100 and 102.5.
Currently there are few to no viable treatment options for these two diseases which is why I so very much hope this is not her problem.
On the off chance that she might has significant internal parasites which are causing her to feel debilitated, I'd worm
her with Pyrantal Paomate (aka Strongid, Nemex, Evict) which can be purchased at most local pet or grain stores. I'd worm her, her brother and mother and any other household cats with whom she comes in contact every two weeks for three doses. This is a very, very wormer and not likely to cause harm if she doesn't have parasites.
I dearly wish I could advise some other treatment options for her which would help her to eat better and to gain weight but until or unless the underlying problem is identified, there's not anything that I feel would be safe to give her.
I'm sorry that my answer is not one which you're likely to find very encouraging and I'm sorry for that; however, I would be doing a disservice to both you and Cowie if I were less than truthful and honest in my response to you. I hope you understand.
I also hope this helps to provide possible explanations for her condition. Again, my profuse apologies for the delayed reply. Deb